This invention relates to diffusion devices for aerating water with small bubbles using a diffusor element arranged over a solid plate-type support element and, more particularly, to such diffusors having fastening devices to connect the diffusor with the support element along the outer edges in such a way that a seal is maintained and, at the same time, the fastening device is removable. The diffusor element contains holes and, when little or no air is supplied, the diffusor element rests tightly on at least one surface of the support element.
The Messner U.S. Pat. No. 4,624,781 describes an air distribution device for aeration of water with small bubbles, especially in biological wastewater treatment plants. This air diffusion device consists of a solid plate and a perforated air diffusion membrane. The edges of the membrane are joined to the solid plate by peripheral strips fastened to the plate to provide for a peripheral seal. The design also has strips within the area of the membrane which are fastened to the solid plate. The solid plate typically is made of plastic or asbestos cement and the sealing strips are made of plastic.
The edge strips which function as sealing elements are necessary to provide a gas-tight connection between the membrane and the solid plate support around the periphery. When the diffusion element has a large area, such as about 1 by 4 meters, the pressure of the gas beneath the diffusor exerts large forces on the connections between the support member and the diffusor. The connection between these edge strips (and sometimes also the other strips) and the membrane with the solid plate is made by self-tapping screws or rivets which penetrate the diffusion member and may weaken it in the region of the screws or rivets. U.S. Pat. No. 4,624,781 also suggests that brackets may be spaced around the edges, to clamp the edge strips and the solid plate together with the membrane between them, but such brackets do not assure a satisfactory seal in the intervals between them. It is also suggested that the brackets may be connected to the solid plate by screws or rivets. In addition, it is known that, for systems which consist of two membranes or a membrane and a plate, a peripheral connection may be accomplished by vulcanizing or using adhesives.
The type of connection between the solid plate, the membrane and edge strips described in U.S. Pat. No. 4,624,781, which also functions as a peripheral seal, is relatively complex with respect to materials, manufacturing and assembly and uncertain in effectiveness. It is, therefore, not a solution which is satisfactory in all respects. In addition, connections between a diffusor and a plate which provide seals by vulcanization or using adhesives are in many cases not possible because the materials used are undesirable for other reasons.